Hey I've got a crazy idea, so here it goes. A few companies sell extended radius arms for ford axles. Why can't you take the arms that are under it, and use the bolt threads on it to screw into a extender bar. Obviously the arm will have to be very beefy, 2x2 square bar .250 thick maybe?

Or would it be better just to weld strait to it? I can fabricate up stuff, so is it worth a shot, or am I just plain nuts? The reason I want to do this is I will be lifting my 79 f150 up at least 9 inches to clear some big tires.

Extending radius arms arms been done more ways than there are to make chicken soup. What you are talking about has been done.. but isn't the strongest way to make them longer. Best way... if your gonna do it yourself... is to peel open the wrapped metal around the bushing stud and have longer ones made. Then you can weld them in place as the factory did.
Myself, I'm looking into a way to make a "modified" four link that would work in place of the radius arms. Way I see it, if both upper and lower links are identical in length or close to identical, your caster change would be as minimal as possible and would make for some wild suspension articulation while also being as durable as possible too. Not to mention, I don't think I have seen anybody do it THAT way yet.
It''s still in the works... and could quite possibly be a reality sometime soon.

I know a buddy who runs a '79 Ford pickup and we have done alot of suspension work to it including lengthening the radius arms. We cut the ends off the arms and sleeved them with a heavy wall rectangle tubing and steel plating. After that we welded on some big Jonny Joints to the end and made new frame mounts. The arms are 16 inches longer and flex like mad even with the stiff six inch lift springs he bought years ago. You have to keep an eye on trac bar bushings unless you run poly or the steering gets twitchy.

Mine on my bronco were 47"s long and had 1-1/4" heims on the end of them. I cut them 6in behind the C-cup on the arm and interlocked some 2"-1/4"wall dom on them. Worked great and flexed like mad. Best set up i had it.

The second one is basically what mine will probably end up looking like. Since my truck is a little tall I was going to make them bend to run along the frame and tuck up a little bit to try to keep the pinion angle from being too bad.

on mine i cut off just the threaded end of the arm and had 13" of 1.25" solid round stock welded on the end then had the threaded end welded back on the solid stock. worked good under my 79 bronco but not fantastic. i had no problems with bending or breaking but i didnt get the flex i wanted. a flex joint of some sorts would have been better. now i have the arms that Fullsize4life had on his bronco. im going to use them with Johnnny Joints on the ends and see what happens.